This section provides background information related to the present disclosure which is not necessarily prior art.
Larger batteries are constructed from individual cells. Usually a battery for hybrid or electric vehicles or industrial applications contains between 20 and several hundred individual cells. The individual cells can thereby be implemented as round cells, as prismatic cells or as so-called Coffee-Bag-cells. Coffee-Bag cells encompass a flexible sleeve, which is made from foil, in which electrical components of the cell are disposed.
Primarily Coffee-Bag cells are used in a battery in order to realize an optimal spatial usage. These furthermore distinguish themselves through low weight with high capacity. Coffee-Bag cells can be cooled well by means of the thermally conductive foils.
Furthermore cells of this construction-type can be easily scaled as all components of the cell, including the foil housing, can be easily changed in size during production.
Because of the high amount of stored energy, larger batteries always represent a safety risk during the occurrence of error functions. In this context lithium batteries are to be viewed particularly critically because they feature a high energy density, a flammable electrolyte, and thin separators. Finally lithium batteries generate high cell voltages so that the components disposed within the cell are exposed to high electro-chemical loads. This is particularly relevant in the case of car and industrial batteries for which lifetimes of at least 8-10 years are scheduled.
The previously mentioned Coffee-Bag cells can be mounted in a space-saving manner. As a result, large amounts of energy can be stored per unit volume in a battery. With this are however substantial construction-driven disadvantages associated. Because of the flexible sleeve the dimension of the Coffee-Bag cells changes when these are charged or discharged. This is also associated with a volume expansion. The volume expansion leads to typical thickness changes of an individual cell of about 5% between charged and uncharged state.
During the assembly of a so-called “stack”, which consists of many individual, switched in series, cells, one must take into account that the individual cells feature a variable volume. It is of particular importance that the cells in the charged state, in which they attain their largest thickness, exert next to no or only minimal pressure onto the surfaces of neighboring cells. In this context it has to be fundamentally considered that the thickness of the flexible cells is, because of manufacturing tolerances, not uniform but subject to variations.
Furthermore there is a need for an arrangement by means of which impacts or vibrations are cushioned and/or dampened so that the interior of the battery as well as contacts do not sustain damage. Furthermore connections of power or monitoring electronics must be connected to the battery for most part free of mechanical loading. A release of only one of the many hundred contacts of the power electronics leads, in the case of a serial circuit to the failure of the battery. In the case of a failure of a contact of the monitoring electronics, the no longer monitored battery can then gradually reach a critical state, which over the intermediate term can lead to the damage or the failure of the entire battery.
The edges of the previously mentioned Coffee-Bag cells feature a sealing seam. This sealing seam joins two foils of a cell which thereby enclose additional components in the hollow space created thereby. For this purpose these foils are coated on the interior side with an electrically isolating, adhesion-promoting seal thermoplastic. This seal thermoplastic can be created from a functionalized polyolefin. This sealing seam represents the mechanical weak point of a Coffee-Bag cell.
Furthermore the air pressure can vary in the surroundings of the cells. If the housing of the battery is hermitically sealed, temperature-dependent pressure variations of typically 0.2 bar can occur. These pressure variations further strain the sealing seams.
The sealing seam represents however also a break-off point that is supposed to provide the electrolyte the opportunity to blow off in the case of a failure of the battery. A bursting of the cell is to be avoided thereby.
If the leaking flammable electrolyte comes in contact with electrodes, it can ignite and lead to fires or explosions. The maximally permitted overpressures in the interior of a Coffee-bag cell are usually significantly less than 0.1 MPa in order to prevent an opening of the sealing seam. The implementation of the current conducting electrodes has to be viewed particularly critically in the case of Coffee-Bag cells. These feature most often a thickness of about 0.1 to 0.3 mm. In this area a possible leakage is particularly critical because leaking electrolyte can instantly ignite at the electrodes. The sealing seam is generally seen as the weak spot of larger cells because it is exposed over years to constant stresses due to cycling.
Finally preferably water-based cooling media are currently used for the cooling of large batteries or, in the context of application with climate-control installation, fluorinated hydrocarbons or carbon dioxide. A direct contact of most cooling media with the interior of the cells can lead to violent chemical reactions. In the case of water-based cooling media hydrogen is for example released, which is easily flammable and can lead to explosions. It is for this reason that a contact cooling is usually employed in the technology, whereby the heat flow between cell and cooling circulation is established via thermally conductive components and the cooling medium as a result does not come into direct contact with the cells.